Ferrous phosphate powders, lithium iron phosphate powders for li-ion battery, and methods for manufacturing the same

ABSTRACT

Ferrous (II) phosphate (Fe 3 (PO 4 ) 2 ) powders, lithium iron phosphate (LiFePO 4 ) powders for a Li-ion battery and methods for manufacturing the same are provided. The lithium iron phosphate powders are represented by the following formula (II):
 
LiFe (1-a) M a PO 4   (II)
 
wherein, M, and a are defined in the specification, the lithium iron phosphate powders are composed of plural flake powders, the length of each of the flake powders is 0.1-10 μm, and a ratio of the length and the thickness of each of the flake powder is in a range from 11 to 400.

CROSS REFERENCE TO RELATED APPLICATION

This application is a continuation-in-part (CIP) of U.S. patentapplication for “Ferrous Phosphate Powders, Lithium Iron PhosphatePowders for Li-Ion Battery and Methods for Manufacturing the Same”, U.S.application Ser. No. 13/908.393 filed Jun. 3, 2013, and the subjectmatter of which is incorporated herein by reference.

U.S. application Ser. No. 13/908,393 filed Jun. 3, 2013 is acontinuation-in-part (CIP) of U.S. application Ser. No. 13/524,287 filedJun. 15, 2012, which claims the benefits of the Taiwan PatentApplication Serial Number 100121234, filed on Jun. 17, 2011.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to ferrous phosphate powders, lithium ironphosphate powders prepared therefrom, and methods for manufacturing thesame. More specifically, the present invention relates to ferrousphosphate powders for preparing Li-ion batteries having large length tothickness ratio, lithium iron phosphate powders prepared therefrom, andmethods for manufacturing the same.

2. Description of Related Art

As the development of various portable electronic devices continues,more and more attention focuses on the techniques of energy storage, andbatteries are the main power supplies for these portable electronicdevices. Among commercial batteries, small-sized secondary batteries areespecially the major power supplies for portable electronic devices suchas cell phones and notebooks. In addition, secondary batteries areapplied to not only portable electronic devices, but also electricvehicles.

Among the developed secondary batteries, the lithium secondary batteries(also named as the Li-ion batteries) developed in 1990 are the mostpopular batteries used nowadays. The cathode material of the initiallithium secondary batteries is LiCoO₂. LiCoO₂ has the properties of highworking voltage and stable charging and discharging voltage, so thesecondary batteries which use LiCoO₂ as a cathode material are widelyapplied to portable electronic devices. Then, LiFePO₄ with an olivinestructure and LiMn₂O₄ with a spinal structure were also developed as acathode material for lithium secondary batteries. Compared to LiCoO₂,the safety of the batteries can be improved, the charge/discharge cyclescan be increased, and the cost can be further reduced when LiFePO4 orLiMn₂O₄ is used as cathode material of secondary batteries.

Although the batteries which use LiMn₂O₄ as cathode materials have lowcost and improved safety, the spinal structure of LiMn₂O₄ may collapseduring the deep discharge process, due to Jahn-Teller effect. In thiscase, the cycle performance of the batteries may further be decreased.When LiFePO₄ is used as cathode material of batteries, the batteriesalso have the properties of low cost and improved safety. In addition,the capacity of LiFePO₄ is higher than that of LiMn₂O₄, so the batteriesmade from LiFePO₄ can further be applied to devices which need largecurrent and high power. Furthermore, LiFePO4 is a non-toxic andenvironmentally friendly material, and also has great high temperaturecharacteristics. Hence, LiFePO₄ is considered as an excellent cathodematerial for lithium batteries. Currently, the average discharge voltageof the lithium batteries using LiFePO₄ as a cathode material is 3.4-3.7V vs. Li⁺/Li.

A conventional structure of the Li-ion batteries comprises: a cathode,an anode, a separator, and a Li-containing electrolyte. The batteriesperform the charge/discharge cycles by the lithium insertion andextraction mechanism, which is represented by the following equations(I) and (II).Charge: LiFePO₄ −xLi⁺ −xe ⁻ →xFePO₄+(1−x)LiFePO₄  (I)Discharge: FePO₄ +xLi⁺ +xe ⁻ →xLiFePO₄+(1−x)FePO₄  (II)

When a charge process of the batteries is performed, Li ions extractfrom the structure of LiFePO₄; and the Li ions insert into the structureof FePO₄ when a discharge process is performed. Hence, thecharge/discharge process of the Li-ion batteries is a two-phase processof LiFePO₄/FePO₄.

Currently, the LiFePO₄ powders are usually prepared by a solid-stateprocess. However, the property of the product is highly related to thesintering temperature of the solid-state process. When the sinteringtemperature is below 700° C., all the raw materials have to be mixedwell. If the raw materials are not mixed well, Fe³⁺ impurity phase willbe present in the LiFePO₄ powders. When sintering temperature is below600° C., the average grain size of the LiFePO₄ powders will be smallerthan 30 μm. However, if the sintering temperature is increased, theaverage grain size of the LiFePO₄ powders will be larger than 30 μm.When the average grain size of the LiFePO₄ powders is larger than 30 μm,a grinding process and a sieving process have to be performed to obtainpowders with specific grain size between 1 μm to 10 μm, in order to beused for preparing Li-ion batteries. Hence, in the case that the LiFePO₄powders are prepared through a solid-state process, the grinding processand the sieving process have to be performed, which may increase thecost of the Li-ion batteries. In addition, the problem of large andnon-uniform grain size of the LiFePO₄ powders may also occur.

Therefore, it is desirable to provide a method for manufacturingmicro-sized, submicro-sized, even nano-sized cathode materials of Li-ionbatteries in a simple way, in order to increase the charge/dischargeefficiency of the batteries and reduce the cost thereof.

SUMMARY OF THE INVENTION

The object of the present invention is to provide ferrous (II) phosphatepowders for manufacturing a cathode material of a Li-ion battery and amethod for manufacturing the same, wherein the ferrous (II) phosphatepowders have nano, micro, or sub-micro grain size and large length tothickness ratio, and can be applied to the current process for preparinglithium iron phosphate powders.

Another object of the present invention is to provide lithium ironphosphate powders for Li-ion batteries and a method for manufacturingthe same, wherein the ferrous (II) phosphate powders of the presentinvention is used to manufacture the lithium iron phosphate powders.Hence, the sintered powders have uniform and small grain size in nano,micro, or sub-micro scale, so the grinding process and the sievingprocess can be omitted. Additionally, the obtained lithium ironphosphate powders have large length to thickness ratio, which canimprove the charge/discharge efficiency of the Li-ion batteries.

To achieve the object, the method for manufacturing ferrous (II)phosphate powders of the present invention comprises the followingsteps: (A) providing a P-containing precursor solution, wherein theP-containing precursor solution comprises: a P-containing precursor; (B)adding a weakly alkaline compound into the P-containing precursorsolution to obtain a mixture; and (C) adding a ferrous (II) compoundinto the mixture to obtain ferrous (II) phosphate powders.

In addition, the present invention also provides ferrous (II) phosphatepowders, which are prepared through the aforementioned method, to applyto prepare cathode materials for Li-ion batteries. The ferrous (II)phosphate powders for manufacturing cathode materials of Li-ionbatteries according to the present invention are represented by thefollowing formula (I):Fe_((3-x))M_(x)(PO₄)₂ .yH₂O  (I)wherein M comprises at least one metal selected from the groupconsisting of Mn, Cr, Co, Cu, Ni, V, Mo, Ti, Zn, Zr, Tc, Ru, Rh, Pd, Ag,Cd, Pt, Au, Al, Ga, In, Be, Mg, Ca, Sr, B, and Nb, 0≦x<1.5, y is anintergral of 0 to 8, the ferrous (II) phosphate powders are composed ofplural flake powders, the length of each of the flake powders is 0.2-10μm, and a ratio of the length and the thickness of each of the flakepowder is in a range from 14 to 500.

In addition, the present invention also provides a method formanufacturing lithium iron phosphate powders for a Li-ion battery,wherein the aforementioned ferrous (II) phosphate powders are used asFe-containing precursors. The method for manufacturing lithium ironphosphate powders of the present invention comprises the followingsteps: (a) providing the aforementioned ferrous (II) phosphate powders;(b) mixing the ferrous (II) phosphate powders with a Li-containingprecursor to obtain mixed powders; and (c) heat-treating the mixedpowders to obtain lithium iron phosphate powders.

When the aforementioned method for manufacturing lithium iron phosphatepowders of the present invention is applied, the obtained lithium ironphosphate powders of the present invention are represented by thefollowing formula (II):LiFe_((1-a))M_(a)PO₄  (II)wherein M comprises at least one metal selected from the groupconsisting of Mn, Cr, Co, Cu, Ni, V, Mo, Ti, Zn, Zr, Tc, Ru, Rh, Pd, Ag,Cd, Pt, Au, Al, Ga, In, Be, Mg, Ca, Sr, B, and Nb, 0≦a<0.5, the lithiumiron phosphate powders are composed of plural flake powders, the lengthof each of the flake powders is 0.1-10 μm, and a ratio of the length andthe thickness of each of the flake powder is in a range from 11 to 400.

The ferrous (II) phosphate powders for manufacturing cathode materialsof Li-ion batteries of the present invention have uniform and smallgrain size in nano, micro, or sub-micro scale, and especially largelength to thickness ratio. However, the grain size of the conventionalferrous (II) phosphate powders or the conventional ferrous phosphateprecursors is large and un-uniform, so the sinter process (i.e. theheat-treating process) has to be performed for at least ten hours, inorder to completely transform the ferrous (II) phosphate powders or theferrous phosphate precursors into lithium iron phosphate. In addition,the grain size of the conventional sintered powders is usually large, soa grinding process and a sieving process have to be performed to obtainpowders with specific size between 1 μm to 1.0 μm. However, the ferrous(II) phosphate powders of the present invention have uniform and smallsize, large length to thickness ratio, and specific shapes. Hence, theferrous (II) phosphate powders can be completely transformed intolithium iron phosphate within several hours (less than 10 hours), so thetime for the sintering process can be greatly reduced. In addition, theobtained lithium iron phosphate powders still have the similar size andthe similar shape as those of the ferrous (II) phosphate powders afterthe sintering process, so the cathode materials of the Li-ion batteriescan be obtained without performing the grinding process and the sievingprocess. Hence, when the ferrous (II) phosphate powders of the presentinvention are used to prepare lithium iron phosphate powders, the timefor the sintering process can be reduced, and the grinding process andthe sieving process can be omitted. Therefore, the cost formanufacturing the Li-ion batteries can be further reduced. In addition,the ferrous (II) phosphate powders of the present invention can bedirectly applied to the current production lines of lithium ironphosphate powders, so it is unnecessary to build new production linesfor manufacturing lithium iron phosphate powders by use of the ferrous(II) phosphate powders of the present invention. Therefore, the cost formanufacturing the Li-ion batteries can be further reduced.

In the ferrous (II) phosphate powders or the lithium iron phosphatepowders of the present invention, the flake powders are powders composedof independent flakes, flake powders that one end of each of the flakepowders connects to each other, flake powders connecting to each otherat the center of the flakes, or flake powders that one end of each ofthe flake powders connects to each other to form a connecting center.More preferably, the flake powders are independent flakes.

In addition, in the ferrous (II) phosphate powders or the lithium ironphosphate powders of the present invention, the thickness of each of theflake powder may be less than 60 nm (for example, 1-60 mm). Preferably,the thickness thereof is 1-50 nm. More preferably, the thickness thereofis 4-45 nm. Most preferably, the thickness thereof is 7-35 nm. Since thethickness of the flake powders is in nano-scale, some of the flakepowders are transparent or semi-transparent.

Furthermore, in the ferrous (II) phosphate powders or the lithium ironphosphate powders of the present invention, the ratio of the length andthe thickness of each of the flake powder may be in a range from 10 to500.

For the ferrous (II) phosphate powders of the present invention,preferably, the ratio of the length and the thickness of each of theflake powder is in a range from 14 to 500. More preferably, the ratiothereof is in a range from 20 to 400. Most preferably, the ratio thereofis in a range from 25 to 250.

For the lithium iron phosphate powders of the present invention,preferably, the ratio of the length and the thickness of each of theflake powder is in a range from 11 to 400. More preferably the ratiothereof is in a range from 30 to 400.

Since the thickness of the ferrous (II) phosphate powders is innano-scale, the sintering time for preparing the lithium iron phosphatepowders can be greatly reduced and the grinding process and a sievingprocess can further be omitted. In addition, since the thickness of thelithium iron phosphate powders is also in nano-scale, thecharge/discharge efficiency of the obtained Li-ion batteries can furtherbe improved.

Furthermore, the ferrous (II) phosphate powders of the present inventionare crystallized ferrous (II) phosphate powders, which may have acrystallization degree of more than 10%.

In addition, the ferrous (II) phosphate powders of the present inventionshows different X-ray diffraction pattern from the conventional ferrous(II) phosphate bulk. More specifically, the ferrous (II) phosphatepowders of the present invention shows an X-ray diffraction pattern 2θangles (°) having characteristic peaks at about 18.32, 19.84, 23.24,28.24, 30.32, 33.34, 35.88, 37.20, 39.36, 40.94, and 41.82. Preferably,the ferrous (II) phosphate powders of the present invention shows anX-ray diffraction pattern 2θ angles (°) having further characteristicpeaks at about 20.72, 22.12, 24.86, 27.08, 34.3, and 44.14.

In the method for manufacturing ferrous (II) phosphate powders of thepresent invention, at least one metal-containing compound may further beadded into the mixture to obtain the doped ferrous (II) phosphatepowders in step (C), and the doped metal in the obtained ferrous (II)phosphate powders can increase the conductivity of the sequentiallyobtained lithium iron phosphate powders. Herein, the molar ratio of themetal-containing compound to the ferrous compound may be 1:1 to 1:999,i.e. the molar content of the metal-containing compound is 0.1-50% ofthat of the ferrous (II) phosphate powders. Preferably, the molar ratioof the metal-containing compound to the ferrous compound is 1:4 to 1:99,i.e. the molar content of the metal-containing compound is 1-20% of thatof the ferrous (II) phosphate powders. In addition, the metal-containingcompound can be any metal salt containing a doped metal of Mn, Cr, Co,Cu, Ni, V, Mo, Ti, Zn, Zr, Tc, R, Rh, Pd, Ag, Cd, Pt, Au, Al, Ga, In,Be, Mg, Ca, Sr, B, or Nb. Preferably, the metal-containing compounds aresulfates, carbonates, nitrates, oxalates, acetates, chlorites, bromides,or iodides of the aforementioned doped metals. More preferably, themetal-containing compounds are sulfates of the aforementioned dopedmetals. Most preferably, the metal-containing compounds are sulfates ofMn, Cr, Co, Cu, Ni, Zn, Al, or Mg.

Hence, in the ferrous (II) phosphate powders and the lithium ironphosphate powders of the present invention, M in the formula (I) andformula (II) respectively comprises at least one metal, which ispreferably selected from the group consisting of Mn, Cr, Co, Cu, Ni, Zn,Al, and Mg. More preferably, the metal is selected from the groupconsisting of Mn, Co, Cu, Zn, Al, Ni, and Mg. In addition, preferably0≦x<0.5 in formula (I). Furthermore, preferably 0≦a<0.15 in formula(II).

The method for manufacturing ferrous (II) phosphate powders of thepresent invention may further comprise a step (C1) after the step (C):washing the ferrous (II) phosphate powders. Herein, the ferrous (II)phosphate powders can be washed with ethanol, water, or a combinationthereof. Preferably, the ferrous (II) phosphate powders are washed withdeionized water. In addition, the method for manufacturing ferrous (II)phosphate powders of the present invention may further comprise a step(C2) after the step (C1): drying the obtained ferrous (II) phosphatepowders. As the temperature of the drying process is increased, the timethereof can be reduced. Preferably, the ferrous (II) phosphate powdersare dried at 40-120° C. for 5-100 hours. More preferably, the ferrous(II) phosphate powders are dried at 50-70° C. for 7-60 hours.

In the ferrous (II) phosphate powders of the present invention, thelength of each of the flake powders may be 0.2-10 μm. Preferably, thelength of each of the flake powders is 0.2-5 μm. More preferably, thelength thereof is 0.3-5 μm. Further preferably, the length thereof is0.4-4 μm. Most preferably, the length thereof is 0.5-4 μm.

In the lithium iron phosphate powders of the present invention, thelength of each of the flake powders may be 0.1-10 μm. Preferably, thelength of each of the flake powders is 0.2-5 μm. More preferably, thelength thereof is 0.3-5 μm. Further preferably, the length thereof is0.4-4 μm. Most preferably, the length thereof is 0.5-4 μm. In addition,the lithium iron phosphate powders of the present invention have olivinestructures.

Furthermore, in the methods for manufacturing the ferrous (II) phosphatepowders and the lithium iron phosphate powders of the present invention,the P-containing precursor can be at least one selected from the groupconsisting of H₃PO₄, NaH₂PO₄, Na₂HPO₄, Mg₃(PO₄)₂, and NH₄H₂PO₄.Preferably, the P-containing precursor is H₃PO₄, NH₃H₂PO₄, or acombination thereof.

In addition, in the methods for manufacturing the ferrous (II) phosphatepowders and the lithium iron phosphate powders of the present invention,the weakly alkaline compound may be at least one selected from the groupconsisting of Na₂CO₃, and NaHCO₃. Preferably, the weakly alkalinecompound is NaHCO₃.

Furthermore, in the methods for manufacturing the ferrous (II) phosphatepowders and the lithium iron phosphate powders of the present invention,the ferrous (II) compound may be at least one selected from the groupconsisting of FeCl₂, FeBr₂, FeI₂, FeSO₄, (NH₄)₂Fe(SO₄)₂, Fe(NO₃)₂,FeC₂O₄, (CH₃COO)₂Fe, and FeCO₃. Preferably, the ferrous compound isFeCl₂, FeSO₄, (NH₄)₂Fe(SO₄)₂, FeCO₃, or a combination thereof. Morepreferably, the ferrous compound is FeSO₄.

In the methods for manufacturing the lithium iron phosphate powders ofthe present invention, the Li-containing precursor may be at least oneselected from the group consisting of LiOH, Li₂CO₃, LiNO₃, CH₃COOLi,Li₂C₂O₄, Li₂SO₄, LiCl, LiBr, LiI, LiH₂PO₄, Li₂HPO₄, and Li₃PO₄.Preferably, the Li-containing precursor is LiOH, Li₂SO₄, LiH₂PO₄, orLi₃PO₄. More preferably, the Li-containing precursor is Li₃PO₄.

In addition, in the methods for manufacturing the lithium iron phosphatepowders of the present invention, the ferrous (II) phosphate powders aremixed with the Li-containing precursor and a carbon-containing materialto obtain mixed powders in step (b). In this case, the surfaces of theobtained lithium iron phosphate powders are coated with carbon, so theconductivity of the obtained lithium iron phosphate powders can furtherbe increased. In addition, the carbon-containing material can alsoinhibit the growth of the lithium iron phosphate powders, so the size ofthe lithium iron phosphate powders can be kept small. Herein, thecarbon-containing material can be any sugar such as sucrose, and also bevitamin C (L-ascorbate). In addition, the additional amount of thecarbon-containing material can be 0.1-40 wt % of the weight of theobtained lithium iron phosphate powders. Preferably the additionalamount of the carbon-containing material is 5-30 wt % of the weight ofthe obtained lithium iron phosphate powders.

In the methods for manufacturing the lithium iron phosphate powders ofthe present invention, the mixed powders can be heat-treated under anatmosphere or with an introduced gas flow to obtain the lithium ironphosphate powders, in step (c). Herein, the atmosphere or the introducedgas flow can be used as a protection gas or a reduction gas, which maycomprise at least one selected from the group consisting of N₂, H₂, He,Ne, Ar, Kr, Xe, CO, methane, N₂—H₂ mixed gas, and a mixture thereof.Preferably, the protection gas or the reduction gas is N₂, H₂, or N₂—H₂mixed gas. More preferably, the protection gas or the reduction gas isN₂—H₂ mixed gas.

Furthermore, in the methods for manufacturing the lithium iron phosphatepowders of the present invention, the mixed powders are heat-treated at300-800° C., preferably. In addition, the mixed powders are preferablyheat-treated for 1-20 hours. More preferably, the mixed powders areheat-treated at 500-750° C. for 1-5 hours.

The obtained lithium iron phosphate powders of the present invention canbe used as cathode materials to prepare Li-ion batteries, through anyconventional method in the art. Here, the method for manufacturing theLi-ion batteries is simply described, but the present invention is notlimited thereto.

An anode and a cathode are provided. Herein, the anode can be a Li-plateor an anode made by a carbon material, which is prepared by coating ananode current collector with a carbon material, and then drying andpressing the carbon material to form an anode for the Li-ion battery.The cathode current collector is coated with a cathode active material(i.e. the lithium iron phosphate powders of the present invention), andthen the cathode active material is dried and pressed to form a cathodefor the Li-ion battery. Next, a separator is inserted between thecathode and the anode, a Li-containing electrolyte is injected, and thena Li-ion battery is obtained after an assembling process.

Other objects, advantages, and novel features of the invention willbecome more apparent from the following detailed description when takenin conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A-1F are SEM photos of ferrous (II) phosphate powders accordingto Embodiment 1 of the present invention;

FIGS. 2A-2B are XRD diffraction patterns of ferrous (II) phosphatepowders according to Embodiment 1 of the present invention;

FIGS. 3A-3B are SEM photos of lithium iron phosphate powders accordingto Embodiment 1 of the present invention;

FIG. 4 is an XRD diffraction pattern of lithium iron phosphate powdersaccording to Embodiment 1 of the present invention;

FIGS. 5A-5B are SEM photos of ferrous (II) phosphate powders doped withMn according to Embodiment 2 of the present invention;

FIGS. 6A-6B are SEM photos of lithium iron phosphate powders doped withMn according to Embodiment 2 of the present invention;

FIGS. 7A-7B are SEM photos of ferrous (II) phosphate powders doped withMg according to Embodiment 3 of the present invention;

FIGS. 8A-8B are SEM photos of lithium iron phosphate powders doped withMg according to Embodiment 3 of the present invention;

FIG. 9 is an XRD diffraction pattern of lithium iron phosphate powdersdoped with Mg according to Embodiment 3 of the present invention;

FIG. 10 is an XRD diffraction pattern of ferrous (II) phosphate powdersdoped with Ni and Mg according to Embodiment 4 of the present invention;

FIG. 11 are XRD diffraction patterns of ferrous (II) phosphate powdersdoped without or with Ni and Mg according to Embodiments 1 and 4 of thepresent invention;

FIGS. 12A-12C are SEM photos of ferrous (II) phosphate powders accordingto Comparative Embodiment of the present invention;

FIG. 13 are XRD diffraction patterns of ferrous (II) phosphate powdersaccording to Embodiments 1 and 5, and Comparative Embodiment of thepresent invention;

FIG. 14 is a SEM photo of lithium iron phosphate powders according toComparative Embodiment of the present invention;

FIGS. 15A-15B are SEM photos of ferrous (II) phosphate powders accordingto Embodiment 5 of the present invention;

FIG. 16 are XRD diffraction patterns of ferrous (II) phosphate powdersaccording to Embodiments 4 and 5 of the present invention;

FIGS. 17A-17D are SEM photos of lithium iron phosphate powders accordingto Embodiment 5 of the present invention;

FIG. 18 is a SEM photo of lithium iron phosphate powders according toEmbodiment 6 of the present invention;

FIG. 19 are XRD diffraction patterns of lithium iron phosphate powdersaccording to Embodiments 1, 5 and 6, and Comparative Embodiment of thepresent invention;

FIG. 20 is a perspective view showing a Li-ion battery according to thepresent invention;

FIG. 21 shows the discharge capacities of Li-ion batteries prepared withlithium iron phosphate powders according to Embodiments 1, 5 and 6, andComparative Embodiment of the present invention;

FIG. 22 shows the discharge capacities of a Li-ion battery prepared withlithium iron phosphate powders according to Embodiment 5 of the presentinvention; and

FIG. 23 shows the relation between the voltage and the specificcapacities of a Li-ion battery prepared with lithium iron phosphatepowders according to Embodiment 5 of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The present invention has been described in an illustrative manner, andit is to be understood that the terminology used is intended to be inthe nature of description rather than of limitation. Many modificationsand variations of the present invention are possible in light of theabove teachings. Therefore, it is to be understood that within the scopeof the appended claims, the invention may be practiced otherwise than asspecifically described.

Embodiment 1 Preparation of Ferrous (II) Phosphate Powders

H₃PO₄ was added in de-ionized water (500 ml) to obtain a P-containingprecursor solution. Next, NaHCO₃ was added into the P-containingprecursor solution to obtain a mixture, in which the molar ratio ofH₃PO₄ to NaHCO₃ was 1:3. After the mixture was stirred for 30 min,ferrous sulfate hydrate (FeSO₄.8H₂O) was added into the mixture, whereinthe molar ratio of FeSO₄.8H₂O to H₃PO₄ was 3:2. After the reaction wascompleted, the product was washed with deionized water, and thencollected with centrifugation for twice. After the collected product wasdried at 55° C. for 45 hours, ferrous (II) phosphate precursor powders(Fe₃(PO₄)₂.8H₂O) were obtained.

The shapes of the ferrous phosphate (II) powders of the presentembodiment were observed with a scanning electron microscope (SEM)(Hitachi S-4000), and the results are shown in FIGS. 1A-1F, which areSEM photos of ferrous (II) phosphate powders observed at themagnification of 1,000×, 5,000×, 10,000×, 10,000×, 20,000× and 50,000×,respectively. As shown in FIG. 1A and FIG. 1B, the ferrous (II)phosphate powders have flat shapes in macro view. As shown in FIG. 1C,parts of the ferrous (II) phosphate powders are formed in independentflakes. As shown in FIG. 1D, flake powders in which one end of each ofthe flake powders connects to each other to form a connecting center arealso observed. Among the observed powders shown in FIG. 1C and FIG. 1D,most of the powders are powders formed in independent flakes; and flakepowders that one end of each of the flake powders connects to each otherto form a connecting center are secondly observed. In addition, as shownin FIG. 1C and FIG. 1D, it can be found that partial flake powders aretransparent or semi-transparent. Furthermore, as shown in FIG. 1E andFIG. 1F, not only transparent or semi-transparent flake powders can beobserved, but also flake powders with partially cured peripheries canalso be found. Furthermore, among the observed ferrous (II) phosphatepowders, the length (L) of each of the flake powders is about 1-4 μm,and most of them is about 1.5-4 μm. The thickness (d) of each of theflake powders is about 10-30 nm. After calculation, the ratio of thelength to the thickness (L/d) is approximately in a range from 50 to400.

In addition, the obtained ferrous (II) phosphate powders of the presentembodiment were also examined with an X-ray diffraction microscope(Shimadzu 6000) to understand the crystal structure thereof. The X-raydiffraction pattern was collected by Cu Kα radiation, the 2θ-scanningangle is 15°-45°, and the scanning rate is 1°/min. The obtained XRDdiffraction pattern is shown in FIGS. 2A-2B, wherein the upper figuresin each figure are the XRD diffraction patterns of ferrous (II)phosphate powders of the present embodiment, the lower figures in eachfigure respectively are the XRD diffraction patterns of ferrous (II)phosphate powders (Fe₃(PO₄)₂.8H₂O) JCPDS Nos. 079-1928 and 083-2453, andlines in the upper figures are used to indicate the position of thepeaks. As shown in FIG. 2A, only one characteristic peak is indicated bysymbol “*”, which indicates that this peak of the ferrous (II) phosphatepowders of the present embodiment corresponds to the peak of ferrous(II) phosphate powders (Fe₃(PO₄)₂.8H₂O) (JCPDS No. 079-1928). Othercharacteristic peaks are indicated by symbols “#”, which indicate thatthese peaks do not correspond to the peaks of JCPDS No. 079-1928; andthere are right shifts of about 0.1°-0.4° between the characteristicpeaks indicated by symbols “#” and the corresponding peaks of JCPDS No.079-1928. Except for the aforementioned characteristic peaks, no otherpeaks are observed in ferrous (II) phosphate powders of the presentembodiment. This result indicates that all the ferrous (II) phosphatepowders prepared according to the present embodiment are indeed ferrous(II) phosphate powders.

In addition, the ferrous (II) phosphate powders of the presentembodiment were observed with a transmission electron microscope (TEM)(JEOL 2010), and the results (data not shown) show that about 10% of thepowders are crystallized ferrous (II) phosphate powders and about 90%thereof are amorphous powders. From the results of SEM and TEM, it canbe inferred that the low signal to noise ratio is caused by the lowcrystallization degree and thin thickness.

Furthermore, the ferrous (II) phosphate powders of the presentembodiment were analyzed with Inductively-coupled plasma massspectrometry (ICP-MS). The results show that the atomic ratio of P to Fe(P/Fe) of the ferrous (II) phosphate powders of the present embodimentis 0.4/0.62=0.645, and that of standard Fe₃(PO₄)₂ powders is ⅔=0.667.This result indicates that the synthesized powders of the presentembodiment has similar atomic ratio P/Fe to that of standard Fe₃(PO₄)₂powders, and the deviation thereof is within the experimental range.

Preparation of Lithium Iron Phosphate Powders

Next, the ferrous (II) phosphate powders of the present embodiment wasused as a precursor, and mixed with Li₃PO₄ in a molar ratio of 1:1. Inaddition, 15 wt % of sugar was also added in the mixture. The mixturewas mixed with a 3D shaker containing 2 mm zirconia balls for 2 hr toobtain mixed powders. Then, the mixed powders were sintered at 750° C.,under an atmosphere of N₂ gas for 3 hrs. Finally, LiFePO₄ powders coatedwith carbon and formed in flake shapes were obtained.

The shapes of the LiFePO₄ powders of the present embodiment wereobserved with a scanning electron microscope (SEM) (Hitachi S-4000), andthe results are shown in FIGS. 3A-3B, which respectively are SEM photosof LiFePO₄ powders observed at the magnification of 20,000× and 50,000×.As shown in FIGS. 3A-3B, the sintered LiFePO₄ powders still have similarshape after the heat-treating process to that of the original ferrous(II) phosphate powders, and most of the powders are powders formed inindependent flakes. In addition, even though the time for performing thesintering process is short, all the ferrous (II) phosphate powders canbe transformed into LiFePO₄, due to the uniform and small size of theferrous (II) phosphate powders. Furthermore, among the observed LiFePO₄powders, the length (L) of each of the flake powders is about 0.6-3 μm,and most of them is about 1-3 μm. The thickness (d) of each of the flakepowders is about 8-20 nm. After calculation, the ratio of the length tothe thickness (L/d) is approximately in a range from 30 to 400. Itshould be noted that the thickness and the length of the LiFePO₄ powdersare smaller than those of LiFePO₄ powders due to the ball milling andsintering process.

Furthermore, the obtained LiFePO₄ powders of the present embodiment werealso examined with an X-ray diffraction microscope (Shimadzu 6000) tounderstand the crystal structure thereof. The X-ray diffraction patternwas collected by Cu Kα radiation, the 2θ-scanning angle is 15°-45°, andthe scanning rate is 1°/min. The obtained XRD diffraction pattern isshown in FIG. 4, wherein the upper figure is the XRD diffraction patternof LiFePO₄ powders of the present embodiment, the lower figure is theXRD diffraction pattern of LiFePO₄ crystal with an olivine structure(JCPDS No. 081-1173), and lines in the upper figure are used to indicatethe position of the peaks.

As shown in FIG. 4, all the lines indicating the peaks of the LiFePO₄powders of the present embodiment correspond to the peaks of LiFePO₄crystal with the olivine structure (JCPDS No. 081-1173), and there areno other peaks observed in the LiFePO₄ powders of the presentembodiment. This result indicates that all the LiFePO₄ powders of thepresent embodiment are LiFePO₄ powders with olivine structures.

Embodiment 2 Preparation of Ferrous (II) Phosphate Powders

H₃PO₄ was added in deionized water (500 ml) to obtain a P-containingprecursor solution. Next, NaHCO₃ was added into the P-containingprecursor solution to obtain a mixture, in which the molar ratio ofH₃PO₄ to NaHCO₃ was 1:3. After the mixture was stirred for 30 min,ferrous sulfate hydrate (FeSO₄.8H₂O) and manganese sulfate hydrate(MnSO₄.5H₂O) was added into the mixture, wherein the molar ratio ofFeSO₄.8H₂O to MnSO₄.6H₂O was 9:1, and the molar ratio of the totalamount of FeSO₄.8H₂O and MgSO₄.6H₂O to H₃PO₄ was 3:2. After the reactionwas completed, the product was washed with deionized water, and thencollected with centrifugation twice. After the collected product wasdried at 55° C. for at least 36 hours, ferrous (II) phosphate precursorpowders doped with Mn (Fe_(2.7)Mn_(0.3)(PO₄)₂.8H₂O) were obtained.

The shapes of the ferrous (II) phosphate precursor powders doped with Mnof the present embodiment were also observed with a scanning electronmicroscope (SEM) (Hitachi S-4000), and the results are shown in FIGS.5A-5B, which respectively are SEM photos of ferrous (II) phosphatepowders observed at the magnification of 10,000× and 50,000×. As shownin FIG. 5A, it was observed that the powders are composed of pluralflake powders. More specifically, the powders are powders composed ofindependent flakes, flake powders that one end of each of the flakepowders connects to each other to form a connecting center, or flakepowders connecting to each other at the center of the flakes. Herein,among the observed ferrous (II) phosphate powders of the presentembodiment, the length (L) of each of the flake powders is about 0.3-3μm, and most of them is about 1-3 μm. The thickness (d) of each of theflake powders is about 10-22 nm. After calculation, the ratio of thelength to the thickness (L/d) is approximately in a range from 15 to300.

Preparation of Lithium Iron Phosphate Powders

Next, the ferrous (II) phosphate powders doped with Mn of the presentembodiment was used as a precursor, and mixed with Li₃PO₄ in a molarratio of 1:1. In addition, 15 wt % of sugar was also added in themixture. The mixture was mixed with a 3D shaker containing 2 mm zirconiaballs for 2 hrs to obtain mixed powders. Then, the mixed powders wereheat-treated at 750° C., under an atmosphere of N₂ gas for 3 hrs.Finally, lithium iron phosphate powders coated with carbon and dopedwith Mn (LiFe_(0.9)Mn_(0.1)PO₄/C) and which were formed in flake shapes,were obtained.

The shapes of the lithium iron phosphate powders doped with Mn of thepresent embodiment were also observed with a scanning electronmicroscope (SEM) (Hitachi S-4000), and the results are shown in FIGS.6A-6B, which respectively are SEM photos of LiFePO₄ powders observed atthe magnification of 20,000× and 50,000×. The result shows that thelithium iron phosphate powders doped with Mn of the present embodimenthave similar shape to that of ferrous (II) phosphate powders doped withMn, and especially most of the powders are powders formed in independentflakes; and flake powders that one end of each of the flake powdersconnects to each other to form a connecting center are secondlyobserved.

Furthermore, among the observed LiFePO₄ powders, the length (L) of eachof the flake powders is about 0.9-3 μm, and most of them is about 1-3μm. The thickness (d) of each of the flake powders is about 7-20 nm.After calculation, the ratio of the length to the thickness (L/d) isapproximately in a range from 45 to 430.

Embodiment 3

The ferrous (II) phosphate precursor powders doped with Mg of thepresent embodiment were prepared through the same process as illustratedin Embodiment 2, except that the MnSO₄.5H₂O was substituted withmagnesium nitrate hydrate (MgN₂O₆.6H₂O) in the present embodiment. Inaddition, the lithium iron phosphate powders doped with Mg of thepresent embodiment were also prepared through the same process asillustrated in Embodiment 2, except that the ferrous (II) phosphateprecursor powders doped with Mn used in the Embodiment 2 weresubstituted with ferrous (II) phosphate precursor powders doped with Mgprepared in the present embodiment.

After preparation, ferrous (II) phosphate precursor powders doped withMg (Fe_(2.7)Mg_(0.3)(PO₄)₂.8H₂O) and lithium iron phosphate powderscoated with carbon and doped with Mg (LiFe_(0.9)Mg_(0.1)PO₄/C) wereobtained.

The shapes of the ferrous (II) phosphate precursor powders and lithiumiron phosphate powders doped with Mg of the present embodiment were alsorespectively observed with a scanning electron microscope (SEM) (HitachiS-4000), and the results thereof are respectively shown in FIGS. 7A-7Band FIGS. 8A-8B. FIGS. 7A and 7B are SEM photos of ferrous (II)phosphate powders of the present embodiment observed at themagnification of 10,000× and 50,000×, respectively; and FIGS. 8A and 8Bare SEM photos of LiFePO₄ powders observed at the magnification of10,000× and 50,000×, respectively.

Among the observed ferrous (II) phosphate powders doped with Mg of thepresent embodiment, the length (L) of each of the flake powders is about0.3-2.5 μm, and most of them is about 1-2.5 μm. The thickness (d) ofeach of the flake powders is about 10-22 nm. After calculation, theratio of the length to the thickness (L/d) is approximately in a rangeof 14-250.

In addition, among the observed LiFePO₄ powders doped with Mg of thepresent invention, the length (L) of each of the flake powders is about0.75-2.5 μm, and most of them is about 1-2.5 μm. The thickness (d) ofeach of the flake powders is about 8-20 nm. After calculation, the ratioof the length to the thickness (L/d) is approximately in a range of40-300.

In addition, the obtained lithium iron phosphate powders doped with Mgof the present embodiment were also examined with an X-ray diffractionmicroscope (Shimadzu 6000) to understand the crystal structure thereof.The X-ray diffraction pattern was collected by Cu Kα radiation, the2θ-scanning angle is 15°-45°, and the scanning rate is 1°/min. Theobtained XRD diffraction pattern is shown in FIG. 9, wherein the upperfigure is the XRD diffraction pattern of lithium iron phosphate powdersdoped with Mg of the present embodiment, the lower figure is the XRDdiffraction pattern of LiFePO₄ crystal with an olivine structure (JCPDSNo. 081-1173), and lines in the upper figure are used to indicate theposition of the peaks. The result indicates that all the lithium ironphosphate powders doped with Mg of the present embodiment have olivinestructures.

Embodiment 4

The ferrous (II) phosphate precursor powders doped with Mg and Ni of thepresent embodiment were prepared through the same process as illustratedin Embodiment 2, except that the MnSO₄.5H₂O was substituted withMgN₂O₆.6H₂O and nickel nitrate hydrate (Ni(NO₃)₂.6H₂O) and the molarratio of FeSO₄.8H₂O:MgN₂O₆.6H₂O:Ni(NO₃)₂.6H₂O was 8:1:1 in the presentembodiment. In addition, the lithium iron phosphate powders doped withMg and Ni of the present embodiment were also prepared through the sameprocess as illustrated in Embodiment 2, except that the ferrous (II)phosphate precursor powders doped with Mn used in the Embodiment 2 weresubstituted with ferrous (II) phosphate precursor powders doped with Mgand Ni prepared in the present embodiment.

After preparation, ferrous (II) phosphate precursor powders doped withMg and Ni (Fe₂.4Mg_(0.3)Ni_(0.3)(PO₄)₂.8H₂O) and lithium iron phosphatepowders coated with carbon and doped with Mg and Ni(LiFe_(0.8)Mg_(0.1)Ni_(0.1)PO₄/C) were obtained.

Among the observed ferrous (II) phosphate powders doped with Mg and Niof the present embodiment, the length (L) of each of the flake powdersis about 0.3-2.7 μm, and most of them is about 1-2.7 μm. The thickness(d) of each of the flake powders is about 10-22 nm. After calculation,the ratio of the length to the thickness (L/d) is approximately in arange of 14-270.

In addition, among the observed LiFePO₄ powders doped with Mg and Ni ofthe present invention, the length (L) of each of the flake powders isabout 0.5-2 μm, and most of them is about 1-1.7 μm. The thickness (d) ofeach of the flake powders is about 7-20 nm. After calculation, the ratioof the length to the thickness (L/d) is approximately in a range of25-300.

The ferrous (II) phosphate powders doped with Ni and Mg of the presentembodiment were analyzed with Inductively-coupled plasma massspectrometry (ICP-MS). The results show that the atomic ratio of P to Fe(P/(Fe+ Ni+Mg)) of the ferrous (II) phosphate powders doped with Ni andMg of the present embodiment is 0.39/(0.042+0.49+0.056)=0.663, and thatof standard Fe₃(PO₄)₂ powders is ⅔=0.667. This result indicates that thesynthesized powders doped with Ni and Mg of the present embodiment hassimilar atomic ratio P/Fe to that of standard Fe₃(PO₄)₂ powders, and thedeviation thereof is within the experimental range.

In addition, the ferrous (II) phosphate powders doped with Ni and Mg ofthe present embodiment were observed with a transmission electronmicroscope (TEM) (JEOL 2010), and the results (data not shown) show thatabout 15% of the powders are crystallized ferrous (II) phosphate powdersand about 85% thereof are amorphous powders.

The obtained ferrous (II) phosphate powders doped with Mg and Ni of thepresent embodiment were also examined with an X-ray diffractionmicroscope (Shimadzu 6000) through the same process as illustrated inEmbodiment 1, to understand the crystal structure thereof. The obtainedXRD diffraction pattern is shown in FIG. 10, wherein the upper figure isthe XRD diffraction pattern of ferrous (II) phosphate powders of thepresent embodiment, the lower figure is the XRD diffraction pattern offerrous (II) phosphate powders (Fe₃(PO₄)₂.8H₂O) JCPDS No. 079-1928.

As shown in FIG. 10, a characteristic peak is indicated by symbols “*”(*13), and there is a 0.04° right shift between this peak of the powdersof the present embodiment and JCPDS standard. Some characteristic peaksare indicated by symbols “♦” (♦8 and ♦14), which indicate that theintensities of these peaks of the powders of the present embodiment aresignificantly increased in comparison to those of JCPDS standard. Theresidue characteristic peaks are indicated by symbols “#”, whichindicate that there are at least a 0.12° right shift between these peaksof the powders of the present embodiment and JCPDS standard, and theintensities of these peaks of the powders of the present embodiment arealso different from intensities of the closest peaks of JCPDS standard,and some of them have weak intensities. The intensities and the 2θangles (°) of the powders of the present embodiment and JCPDS standardare summarized in the following Table 1, in which I indicates the peakintensity of the JCPDS standard and the relative intensity of theobserved characteristic peaks of the powders of the present embodiment,wherein “vs” stands for very strong, “s” for strong, “o” for ordinary,“w” for weak, and “vw” stands for very weak.

TABLE 1 JCPDS Card Data 079-1928 peak 2θ (°) I 2θ (°) I [%] #1 18.32 vs18.14 24.5 #2 19.84 o 19.39 11.3 #3 20.72 vw 20.38 4.2 #4 22.12 vw 21.8110.3 22.48 0.3 #5 23.24 s 23.10 18.4 23.51 1 #6 24.86 vw 24.34 3.9 26.500.3 #7 27.08 vw 26.68 2.1 27.76 26.7 #8 28.24 s 28.17 2.3 29.85 19.1 #930.32 vs 30.20 13.4 30.84 0.9 32.31 3.5 32.75 17.5 #10 33.34 s 33.0514.4 #11 34.3 vw 34.00 8.7 34.58 3 35.44 10.9 #12 35.88 w 35.74 5.336.76 0.8 *13 37.20 o 37.16 13.4 38.24 0.3 38.76 9.8 39.11 1 39.21 1.3#14 39.36 w 39.36 0.7 39.59 3.1 40.34 8.4 #15 40.94 w 41.11 7.6 41.442.9 #16 41.82 w 41.68 1.2 42.87 1.7 43.29 1.4 43.62 6.9 #17 44.14 vw44.45 0.4 45.04 2.3

It is known that there might be left shifts of the peaks in the X-raydifferaction pattern but the relative intensities thereof are maintainedwhen the lattice constant of the crystal is slightly changed. However,as shown in FIG. 10 and Table 1, there are not only the shifts but alsothe intensity changes observed in the synthesized ferrous (II) phosphateof the present embodiment. Hence, it can be concluded that the shiftsand the intensity changes observed in the synthesized powder of thepresent embodiment is not only caused by the slight change of thelattice constant of the ferrous (II) phosphate, and the crystalstructure thereof is different from that of the standard ferrous (II)phosphate.

In addition, FIG. 11 are XRD diffraction patterns of ferrous (II)phosphate powders doped without or with Ni and Mg according toEmbodiments 1 and 4 of the present invention, in which the upper andlower figures are respectively the patterns of powders prepared inEmbodiments 1 and 4. It can be found that the intensities and the 20degrees of the powders prepared in Embodiments 1 and 4 are almost thesame. Especially, the signal to noise ratio of Embodiment 4 is higherthan that of Embodiment 1, and it is because the crystallization degreeof the powders of Embodiment 4 is higher than that of Embodiment 1.

Comparative Embodiment Preparation of Ferrous (II) Phosphate Powders

H₃PO₄ and NaHCO₃ were mixed in a molar ratio of 1:3, and dissolved indeionized water (200 ml) to obtain a P-containing precursor solution.After the P-containing precursor solution was stirred for 30 mins,ferrous sulfate hydrate (FeSO₄.8H₂O) was added into the P-containingprecursor solution, wherein the molar ratio of FeSO₄.8H₂O to H₃PO₄ was3:2. After the reaction was completed, the product was washed with anethanol solution, and then collected with centrifugation for twice.After the collected product was dried at 60° C. for 12 hours, ferrous(II) phosphate precursor powders (Fe₃(PO₄)₂.8H₂O) were obtained.

The shapes of the ferrous phosphate (II) powders of the presentembodiment were observed with a scanning electron microscope (SEM)(Hitachi S-4000), and the results are shown in FIGS. 12A-12C, which areSEM photos of ferrous (II) phosphate powders observed at themagnification of 1,000×, 10,000×, and 10,000×, respectively. As shown inFIG. 12A, the ferrous (II) phosphate powders have flat shapes in macroview. As shown in FIG. 12B, powders composed of independent flakes, andflake powders that one end of each of the flake powders connects to eachother to form a connecting center can be observed. As shown in FIG. 12C,flake powders connecting to each other at the center of the flakes canalso be observed. Among the observed powders, most of the powders areflake powders that one end of each of the flake powders connects to eachother to form a connecting center. In addition, among the observedferrous (II) phosphate powders, the length (L) of each of the flakepowders is about 3-5 μm. The thickness (d) of each of the flake powdersis about 120-165 nm, and most of them is about 120-130 nm. Aftercalculation, the ratio of the length to the thickness (L/d) isapproximately in a range from 18 to 42.

Herein, the X-ray diffraction pattern thereof was also collected throughthe same process as those described in Embodiment 1, and the resultthereof is shown in FIG. 13, wherein the lines shown in the figureindicate the position of the peaks of ferrous (II) phosphate powdersJCPDS No. 079-1928. All the peaks observed in the powders of the presentcomparative embodiment correspond to those of JCPDS, and there are noother peaks observed in the powders of the present comparativeembodiment. This result indicates that all the ferrous (II) phosphatepowders prepared according to the present embodiment are indeed ferrous(II) phosphate powders.

Preparation of Lithium Iron Phosphate Powders

Next, the ferrous (II) phosphate powders of the present comparativeembodiment was used as a precursor, and mixed with Li₃PO₄ in a molarratio of 1:1. In addition, 15 wt % of sugar was also added in themixture. The mixture was mixed with a 3D shaker containing zirconiaballs (1 cm) for 2 hr to obtain mixed powders. Then, the mixed powderswere heat-treated at 750° C., under an atmosphere of N₂ gas for 3 hrs.Finally, LiFePO₄ powders coated with carbon and which were formed inflake shape, were obtained.

The shapes of the LiFePO₄ powders of the present embodiment wereobserved with a scanning electron microscope (SEM) (Hitachi S-4000), andthe results are shown in FIG. 14, which is a SEM photo of LiFePO₄powders observed at the magnification of 20,000×. It can be found thatthe sintered LiFePO₄ powders still have similar shape after theheat-treating process to that of the original ferrous (II) phosphatepowders, and most of the powders are flake powders that one end of eachof the flake powders connects to each other to form a connecting center.Furthermore, among the observed LiFePO₄ powders, the length (L) of eachof the flake powders is about 1.2-2 μm, and most of them is about1.5-1.8 μm. The thickness (d) of each of the flake powders is about35-140 nm, and most of them is about 100-1.30 nm.

Embodiment 5 Preparation of Ferrous (II) Phosphate Powders

The ferrous (II) phosphate precursor powders of the present embodimentwere prepared through the same process as illustrated in Embodiment 1,except that ferrous sulfate hydrate (FeSO₄.8H₂O) was added slowly intothe mixture, and the collected product was dried at 55° C. for 36 hours.The obtained ferrous (II) phosphate precursor powders have differentlength and thickness.

The shapes of the ferrous phosphate (II) powders of the presentembodiment were observed with a scanning electron microscope (SEM)(Hitachi S-4000), and the results are shown in FIGS. 15A-15B, which areSEM photos of ferrous (II) phosphate powders observed at themagnification of 10,000×, and 50,000×, respectively. As shown in FIG.15A, the ferrous (II) phosphate powders have flat shapes in macro view.As shown in FIG. 15B, powders composed of independent flakes and flakepowders in which one end of each of the flake powders connects to eachother to form a connecting center are observed. Among the observedpowders shown in FIGS. 15A-15B, most of the powders are powders formedin independent flakes; and flake powders that one end of each of theflake powders connects to each other to form a connecting center aresecondly observed. Furthermore, among the observed ferrous (II)phosphate powders, the length (L) of each of the flake powders is about0.9-2.6 μm, and most of them is about 1.5-2 μm. The thickness (d) ofeach of the flake powders is about 29-35 nm. After calculation, theratio of the length to the thickness (L/d) is approximately in a rangefrom 25 to 90.

Herein, the X-ray diffraction pattern thereof was also collected throughthe same process as that described in Embodiment 1, and the resultthereof is shown in FIG. 16, wherein the lines shown in the figureindicate the position of the peaks of ferrous (II) phosphate powdersJCPDS No. 079-1928. As shown in FIG. 16, only one characteristic peak isindicated by the symbol “*”, which indicates that this peak of thepowders of the present embodiment corresponds to the peak of ferrous(II) phosphate powders (Fe₃(PO₄)₂.8H₂O) (JCPDS No. 079-1928). Othercharacteristic peaks observed in the powders of the present embodimentcannot consistently correspond to those of JCPDS but have similar 2θangles to those observed in the powders of Embodiment 4. This resultindicates that the crystal structure of the powders of Embodiment 4 andthe present embodiment is different from that of the standard ferrous(II) phosphate (JCPDS No. 079-1928).

In addition, as shown in FIG. 13, the signal to noise ratio of thepresent embodiment is higher than that of Embodiment 1, and it isbecause the thickness of the powders of the present embodiment is largerthan that of Embodiment 1.

Preparation of Lithium Iron Phosphate Powders

The lithium iron phosphate powders of the present embodiment were alsoprepared through the same process as illustrated in ComparativeEmbodiment, except that the ferrous (II) phosphate powders of theComparative Embodiment was substituted with those of the presentembodiment, and the 1 cm zirconia balls were substituted with 2 mmzirconia balls.

The shapes of the LiFePO₄ powders of the present embodiment wereobserved with a scanning electron microscope (SEM) (Hitachi S-4000), andthe results are shown in FIGS. 17A-17D, which are SEM photos of LiFePO₄powders observed at the magnification of 50,000×. As shown in FIGS.17A-17D, the sintered LiFePO₄ powders still have similar shape after theheat-treating process to that of the original ferrous (II) phosphatepowders. As shown in FIGS. 17A-17D, flake powders with partially curedperipheries can be found, some of the flake powders are transparent orsemi-transparent, and most of the powders are powders formed inindependent flakes. In addition, even though the time for performing thesintering process is short, all the ferrous (II) phosphate powders canbe transformed into LiFePO₄, due to the uniform and small grain size ofthe ferrous (II) phosphate powders. Furthermore, among the observedLiFePO₄ powders, 90% of them have flake shapes and the thickness thereofis thicker than that of ferrous (II) phosphate powders. In addition, thelength (L) of each of the flake powders is about 0.5-1.5 μm, and most ofthem is about 0.5-1 μm. The thickness (d) of each of the flake powdersis about 34-45 nm. After calculation, the ratio of the length to thethickness (L/d) is approximately in a range from 11 to 45. Furthermore,10% of the flake powder has a thickness of about 15 nm.

Embodiment 6 Preparation of Lithium Iron Phosphate Powders

The lithium iron phosphate powders of the present embodiment were alsoprepared through the same process as illustrated in ComparativeEmbodiment, except that the ferrous (II) phosphate powders of theComparative Embodiment was substituted with those of Embodiment 1, andthe 1 cm zirconia balls were substituted with 0.8 mm zirconia balls.

The shapes of the LiFePO₄ powders of the present embodiment wereobserved with a scanning electron microscope (SEM) (Hitachi S-4000), andthe results are shown in FIG. 18, which is a SEM photo of LiFePO₄powders observed at the magnification of 50,000×.

As shown in FIG. 18, the sintered LiFePO₄ powders still have similarshape after the heat-treating process to that of the original ferrous(II) phosphate powders. As shown in FIG. 18, flake powders withpartially cured peripheries can be found, some of the flake powders aretransparent or semi-transparent, and most of the powders are powdersformed in independent flakes. In addition, even though the time forperforming the sintering process is short, all the ferrous (II)phosphate powders can be transformed into LiFePO₄, due to the uniformand small grain size of the ferrous (II) phosphate powders. Furthermore,among the observed LiFePO₄ powders, the length thereof is shorter thanthat of ferrous (II) phosphate powders, and the thickness thereof isthinner than that of ferrous (II) phosphate powders. In addition, thelength (L) of each of the flake powders is about 0.2-1.7 μm, and most ofthem is about 0.4-1.1 μm. The thickness (d) of each of the flake powdersis about 7-9 nm. After calculation, the ratio of the length to thethickness (L/d) is approximately in a range from 20 to 250.

In addition, the X-ray diffraction patterns of LiFePO₄ powders obtainedin Embodiments 5 and 6, and Comparative Embodiment were also collectedthrough the same process as that described in Embodiment 1, and theresult thereof is shown in FIG. 19, wherein the lines shown in thefigure indicate the position of the peaks of LiFePO₄ (JCPDS No.081-1173). As shown in FIG. 19, all the lines indicating the peaks ofthe LiFePO₄ powders of Embodiments 5 and 6, and Comparative Embodimentcorrespond to the peaks of LiFePO₄ crystal with the olivine structure(JCPDS No. 081-1173), and there are no other peaks observed in theLiFePO₄ powders of Embodiments 5 and 6, and Comparative Embodiment. Thisresult indicates that all the LiFePO₄ powders of the Embodiments 5 and6, and Comparative Embodiment are LiFePO₄ powders with olivinestructures.

According to the results of Embodiments 1-5, the ferrous (II) phosphatepowders have small and uniform grain size. When these ferrous (II)phosphate powders are used as a precursor for preparing lithium ionphosphate powders, the time for the heat-treating process can beshortened. Hence, the cost for manufacturing the Li-ion batteries can befurther reduced. In addition, the sintered lithium ion phosphate powdershave similar shape to that of ferrous (II) phosphate powders, so thesintered lithium ion phosphate powders also have small and uniform grainsize. Hence, the grinding process and the sieving process can be omittedduring the process for preparing the cathode materials, so the cost ofLi-ion batteries can be reduced. Furthermore, the lithium iron phosphatepowders of the present invention have nano, micro, or sub-micro grainsize. When the lithium iron phosphate powders of the present inventionare used as cathode materials of Li-ion batteries, the Li-ion batteriescan exhibit uniform charging and discharging current, and excellentcharge/discharge efficiency. Hence, not only the cost of the Li-ionbatteries can be reduced, but also the charge/discharge time can beshortened and the capacity of the batteries can be further improved.

Preparation and Testing of Li-Ion Batteries

The Li-ion battery of the present invention was prepared through theconventional manufacturing method thereof. Briefly, PVDF, LiFePO₄prepared in Embodiments 1, 5-6, or Comparative Embodiment of the presentembodiment, ZrO, KS-6 [TIMCAL] and Super-P [TIMCAL] were dried in avacuum oven for 24 hr, and a weight ratio of LiFePO₄:PVDF:KS-6:Super-Pwas 85:10:3:2. Next, the aforementioned materials were mixed with a 3Dmiller containing NMP to obtain slurry. An Al foil was provided andcoated with the slurry through a blade coating process, and then placedin a vacuum oven at 90° C. for 12 hr. The dried foil coated with theslurry was pressed by a roller, and cut into Φ13 mm circular plates.

Next, as shown in FIG. 20, an upper cap 17, a lower cap 11, a widemouthplate 16, a pad 15, the aforementioned circular plate 12 with the slurrycoated on a surface 121 thereof, and a Φ18 mm separator 13 are placed ina vacuum oven at 90° C. for 24 hr, and then placed into a glove box withless than 1 ppm of water and O₂ under Ar environment. After immersingthe circular plate 12, and the separator 13 with electrolyte, thecircular plate 12, the separator 13, a Li-plate 14, the pad 15, thewidemouth plate 16 and the upper cap 17 were sequentially laminated onthe lower cap 11, as shown in FIG. 20. After pressing and sealing, aCR2032 coin type Li-ion battery was obtained, and tested after 12-30 hr.

The obtained Li-ion batteries prepared by LiFePO₄ of Embodiments 1, 5-6,or Comparative Embodiment were tested with automatic cellcharge-discharge test system (AcuTech Systems BAT-750B). First, thebatteries were activated, and charged with constant voltage 3.65V 0.1 C.When the charge current was less than 0.02 mA or the charging capacityreached 2 mAh, the batteries were discharged with constant current 0.1 Cuntil the voltage thereof was 2V. After the aforementioned steps wereperformed for several times, the constant voltage for charging wasincreased to 3.9V, and the other conditions were maintained. After thesteps were performed using the constant voltage of 3.9V for severaltimes, the constant voltage for charging was further increased to 4.2 V,and the other conditions were maintained. After the steps were performedusing the constant voltage of 4.2V for several times, the chargingcurrent was sequentially increased to 0.2 C, 0.5 C, 0.75 C and 1 C, andthe other conditions were maintained. The batteries charged withdifferent charge current were tested after the batteries were chargedand discharged for several times.

After the Li-ion batteries were charged with constant voltage 4.2V, 0.75C, and the discharge testing were performed at three different constantcurrent (0.1 C, 0.2 C and 0.5 C) when the charge current was less than0.02 mA. The discharge was set to stop when the voltage of the constantcurrent discharge was 2V, and each discharge current was tested by twocharge/discharge cycles. The results thereof are shown in FIG. 21.

As shown in FIG. 21, the discharge capacities of the batteries preparedwith the LiFePO₄ of Comparative Embodiment (thickness=100-130 nm)decreased from 83 mAh/g of 0.1 C discharge current, 81 mAh/g of 0.2 Cdischarge current, to 76 mAh/g of 0.5 C discharging current. Thedischarge capacities of the batteries prepared with the LiFePO₄ ofEmbodiment 5 (thickness=34-45 nm) decreased from 131 mAh/g of 0.1 Cdischarge current, 130 mAh/g of 0.2 C discharge current, to 121 mAh/g of0.5 C discharge current. The discharge capacities of the batteriesprepared with the LiFePO₄ of Embodiment 1 (thickness=8-20 nm) decreasedfrom 152 mAh/g of 0.1 C discharge current, 138 mAh/g of 0.2 C dischargecurrent, to 129 mAh/g of 0.5 C discharge current. In addition, thedischarge capacities of the batteries prepared with the LiFePO₄ ofEmbodiment 6 (thickness=7-9 nm) decreased from 164 mAh/g of 0.1 Cdischarge current, 158 mAh/g of 0.2 C discharge current, to 145 mAh/g of0.5 C discharge current.

Although the XRD patterns of the LiFePO₄ of Embodiments 1, 5-6, andComparative Embodiment are almost the same (as shown in FIG. 19), theLi-ion batteries prepared with the same have different performances (asshown in FIG. 21). These results indicate that the performances of thebatteries are highly related to the sizes and the shapes of the LiFePO₄powders.

It should be noted that the LiFePO₄ powders of Embodiment 6 has thesmallest thickness and length, and the Li-ion batteries prepared withthe same has the highest specific capacity Especially, the specificcapacity thereof under 0.1 C discharge current was about 164 mAh/g,which is close to the theoretical value of 170 mAh/g; and showed betterperformance than that prepared with LiFePO₄ of Comparative Embodiment(83 mAh/g under 0.1 C discharge current). These results indicate thatthe specific capacities of the Li-ion batteries are highly related tothe thickness of the LiFePO₄ flake powders, and the specific capacitiesthereof are increased as the thickness of the powders decreased.

FIG. 22 shows the specific capacities of a Li-ion battery prepared withlithium iron phosphate powders according to Embodiment 5 of the presentinvention, wherein the solid dots and the circles respectively indicatethe discharge capacities under 0.1 C and 0.2 C. The discharge current ofdifferent cycle may be varied. The charge current was 0.1 C, 0.2 C, 0.5C, 0.75 C or 1 C and the discharge current was mostly 0.1 C at 1-50cycles and after 170 cycles. The charge current was mostly 0.75 C andthe discharge current was mostly 0.2 C at 51-170 cycles and 375-390cycles. As shown in FIG. 22, there is no significant decay of specificcapacities observed in the Li-ion battery prepared with the LiFePO₄ ofEmbodiment 5 after charge and discharge for 390 cycles.

FIG. 23 shows the relation between the voltage and the specificcapacities of a Li-ion battery prepared with lithium iron phosphatepowders according to Embodiment 5 of the present invention, which wastested by the same charge and discharge current (0.1 C, 0.2 C, 0.5 C,0.75 C and 1 C) at 27-36 cycles. From the results shown in FIG. 23, itcan be found that the voltages of the batteries can be maintained duringthe charge and discharge process, and there are no significantdifference observed between the voltages of the charge and the dischargeprocess. These results indicate the polarization is not significant inthe Li-ion battery prepared with lithium iron phosphate powdersaccording to Embodiment 5 of the present invention.

In conclusion, the ferrous (II) phosphate powders of the presentinvention have thin thickness, and high length to thickness ratio.Hence, the time for preparing LiFePO₄ powders can be greatly reduced. Inaddition, when the obtained LiFePO₄ powders are further applied toprepare Li-ion batteries, the performance of the batteries can begreatly improved.

Although the present invention has been explained in relation to itspreferred embodiment, it is to be understood that many other possiblemodifications and variations can be made without departing from thespirit and scope of the invention as hereinafter claimed.

What is claimed is:
 1. Lithium iron phosphate powders for a Li-ionbattery, represented by the following formula (II):LiFe_((1-a))M_(a)PO₄  (II) wherein M comprises at least one metalselected from the group consisting of Mn, Cr, Co, Cu, Ni, V, Mo, Ti, Zn,Zr, Tc, Ru, Rh, Pd, Ag, Cd, Pt, Au, Al, Ga, In, Be, Mg, Ca, Sr, B, andNb, 0≦a<0.5, the lithium iron phosphate powders are composed of pluralflake powders, the length of each of the flake powders is 0.1-10 μm, thethickness of each of the flake powder is 1-20 nm, and a ratio of thelength and the thickness of each of the flake powder is in a range from11 to
 400. 2. The lithium iron phosphate powders as claimed in claim 1,wherein the flake powders are powders composed of independent flakes,flake powders that one end of each of the flake powders connects to eachother, flake powders connecting to each other at the center of theflakes, or flake powders that one end of each of the flake powdersconnects to each other to form a connecting center.
 3. The lithium ironphosphate powders as claimed in claim 2, wherein the flakes in the flakepowders are independent flakes.
 4. The lithium iron phosphate powders asclaimed in claim 2, wherein one end of each of the flake powdersconnects to each other to form a connecting center.
 5. The lithium ironphosphate powders as claimed in claim 1, wherein the lithium ironphosphate powders have olivine structures.
 6. The lithium iron phosphatepowders as claimed in claim 1, wherein the metal is selected from thegroup consisting of Mn, Cr, Co, Cu, Ni, Zn, Al, and Mg.
 7. The lithiumiron phosphate powders as claimed in claim 1, wherein surfaces of thelithium iron phosphate powders are coated with carbon.
 8. The lithiumiron phosphate powders as claimed in claim 1, wherein 0≦a<0.15.
 9. Thelithium iron phosphate powders as claimed in claim 1, wherein the lengthof each of the flake powders is 0.2-5 μm.
 10. The lithium iron phosphatepowders as claimed in claim 1, wherein the length of each of the flakepowders is 0.4-4 μm.